1. Field of the Invention
The present invention relates to a track locking method for an optical disk drive and an apparatus thereof and more particularly to a method for optical disk drives to execute tracking control through holding the level of a tracking servo output (TRO) signal.
2. Description of the Related Art
In the design of the control chip for an optical disk drive, the track locking control of the pickup head of the optical disk drive is a very important control mechanism. Regardless of the manners of so-called static tracking-on or the track locking process after seeking, they all directly affect the data accessing speed and quality of the optical disk drive.
FIG. 1 is a schematic assembly diagram of the movement elements of a conventional optical disk drive. In FIG. 1, an optical disk 80 has its rotation speed controlled by a spindle motor 11, and on a moving platform 13 seeking motions of an optical pick-up head 12 take place in the radial direction of the optical disk 80 using a sledge motor 14. The sledge motor 14 receives a sledge motor control signal FMO as its driving signal. The optical pick-up head 12 is driven by a tracking servo output signal in order to perform tracking control, or, in other words, moving the laser spot for data access from an adjoining track into the groove 81 of a target track.
In general, during the process of seeking and track locking performed on an optical disk, a track error (TE) signal and an RF ripple (RFRP) signal are two sine waveforms or triangular waveforms with approximately 90° phase differences between each other. As shown in FIG. 2, a digitized TE Zero Cross (TEZC) signal can be figured out if the slice level of the track error signal is treated as a threshold, and similarly digitized RF ripple Zero Cross (RFZC) signal can be figured out if the slice level of the RF ripple is treated as a threshold. An optical disk drive calculates the number of tracks to be sought by the optical pick-up head 12, using the TEZC signal and the RFZC signal. In FIG. 2, the waveforms of the TEZC signal and the RFZC signal between t1 and t3 on the time axis are in response to the relative position of the optical pick-up head 12 inside a groove during an on-track interval, whereas the segments of the waveform that flank the on-track interval are separately on two off-track intervals.
Patent No. 509,913 of the Republic of China discloses a track locking method for an optical disk drive, wherein the track locking control is executed in the following way: a track error signal is held or reshaped at its peak instead of the initial track error signal with a sine wave locus when the optical pick-up head just enters an off-track interval from an on-track interval; thus the optical pick-up head of the optical disk drive can rapidly get to its target track. However, such a holding or reshaping method has a drawback, that is, if the optical disk drive runs at a high rotation speed (greater than 5000 rpm) or a considerable eccentricity (greater than 70 μm) exists, the required number of followed tracks increases and the tracking duration becomes longer, and in consequence the performance of the servo control system of the optical disk drive is unsatisfactory.